In the fabrication of large-scale integrated circuits (LSIs), very fine semiconductor patterns are exposed on a semiconductor substrate. With increasing integration density, the problem of resolution that can be achieved by the optical system used for the exposure is becoming a serious problem.
The desired improvement of the resolution can be achieved when a radiation having a smaller wavelength is employed. Thus, there is an intensive study conducted on the electron beam exposure systems or X-ray exposure systems. In the X-ray exposure systems, there is a problem of relatively low throughput because of the essential nature of this process that the semiconductor pattern has to be written on the substrate in one stroke. In the X-ray exposure systems, on the other hand, there is a problem of constructing a radiation source that has a sufficient output power. Further, there are various problems to be solved, and these exposure processes are not yet used for the mass production of integrated circuits.
In order to achieve the high throughput, it is desirable to use the optical exposure process called aligner wherein an optical radiation such as ultraviolet light is used for the exposure. It should be noted that, in the optical exposure system that has a limitation in the wavelength of the radiation, the resolution of an optical system can be increased by increasing the numerical aperture. On the other hand, it is well known that such an increase in the numerical aperture inevitably invites a decrease in the focal depth. In other words, one has to focus closely the optical beam on the surface of the semiconductor substrate.
Generally, the surface of the semiconductor substrate or wafer is not completely flat but certain deviation from the ideal flatness cannot be avoided. In the usual step-and-repeat process, a region having a size of about 20 mm.times.20 mm is exposed at a time while moving the wafer stepwise in the lateral direction. In such an area of the semiconductor substrate, a surface undulation typically of the magnitude of 0.7-0.8 .mu.m exists.
FIG. 1 shows the example of the surface undulation of the semiconductor substrate. It should be noted that the surface of the semiconductor substrate represented by a line 100 is undulated with a magnitude .DELTA. that is typically of about 0.7-0.8 .mu.m for the unit exposure region designated as EXPOSURE REGION. As already noted, the region typically has the size of about 20 mm.times.20 mm. On the other hand, the image of the semiconductor pattern is formed on a plane represented by f1. In the conventional low-resolution exposure process, such a magnitude of the surface undulation has been within the focal depth of the optical system and the exposure for the entire region could be made without problem. On the other hand, when the submicron patterning process is applied with the optical system having an increased numerical aperture, the focusing control with deviation much smaller than the foregoing magnitude of undulation has to be observed for achieving proper focusing of the semiconductor image on the substrate. Otherwise, the image on the substrate would be blurred.
In order to eliminate the foregoing problem, Japanese Laid-open Patent Application 63-12134 as well as Japanese Laid-open Patent Applications 61-184829, 61-232615 and 61-232616 describe an optical exposure system, wherein the optical radiation produced by an optical source is shaped to form a narrow optical beam that illuminates a limited area of the exposure region shown in FIG. 1, and the narrow optical beam thus formed is scanned over the exposure region while maintaining the focusing of the optical beam on the surface of the semiconductor substrate. In order to form the narrow optical beam, an aperture plate is used. The scanning of the optical beam is caused either by deflecting the optical beam by tilting a mirror that deflects the optical beam to the substrate or by moving the mirror in the direction parallel to the surface of the substrate.
According to the conventional approach disclosed therein, a close focusing of the optical beam is certainly achieved. However, associated with the deflection caused by the tilting of the mirror, the optical beam enters into a demagnifying optical system with a path oblique to the optical axis and there occurs the problem of aberration. When the scanning is achieved by moving the mirror parallel, on the other hand, the effective optical length is changed and one needs additional complex optical system to maintain the focusing of the optical beam on the substrate.